This is a revised application to study the effects of vascular endothelial growth factor (VEGF) in vascular permeability and disease. VEGF induces angiogenesis and is produced in response to hypoxia during ischemic disease and cancer, yet is unique among angiogenic growth factors since it promotes vascular permeability (VP). Recent clinical studies demonstrate that an antibody against VEGF has anti-tumor activity leading to the approval of the first anti-angiogenic drug for cancer patients. While it is believed that the clinical effects of anti-VEGF are based on its anti-angiogenic activity, this has not been established. In preliminary studies, we have developed evidence for an alternative mechanism that may account for some of the anti-VEGF effects in cancer patients. In a recent study, we were able to link the VP promoting effects of VEGF to the ability of tumor cells to form pulmonary and hepatic metastatic lesions in mice. Mechanistically, this process depends, in part, on endothelial cell Src family kinases, since mice deficient in pp60Src or pp62Yes which lack a VEGF-mediated VP response are resistant to metastatic disease. We propose that in the presence of VEGF, Src-deficient mice are unable to regulate the endothelial junctional protein VE-cadherin. To this end, we determined that VE-cadherin, from WT but not Src deficient mice, was phosphorylated in response to VEGF. Our studies suggested that Src-dependent phosphorylation of VE-cadherin caused it to lose adhesive function and dissociate from the cytosolic proteins B-catenin and p120. In this proposal, we will evaluate the molecular pathway involving VEGF regulation of the endothelial cell barrier both in vitro and in vivo. In Aims 1 and 2 we will address this at the molecular level by characterizing how VEGF-mediated signaling activates Src and how Src activity regulates VE-cadherin function. In Aim 3 we will apply this knowledge to study how VEGF regulation of endothelial barrier function impacts tumor cell extravasation and metastasis. These studies not only offer new insight into the role that VEGF and Src play in vascular biology and cancer but suggest a new therapeutic strategy to block VEGF-mediated vascular permeability as a means to regulate metastatic disease.